When large diameter, relatively stiff pipes are lowered into an undersea trench, the usual elastic curvature is such that an unacceptably long span develops, and the corresponding open section of the trench may be too long to be practical unless stable slopes have been cut. For example, if a 12-foot deep trench were in a sandy soil, the slope of the trench sidewalls might be on the order of 1 in 3, and the amount of soil to be excavated would be five times that of a vertical sidewall ditch. Accordingly, it is not only desirable to reduce the volume of the soil to be excavated, but further to prevent infilling of the soil until the pipe has reached the bottom of the ditch, both to reduce power requirements and save time in construction.
Applicants are not aware of any prior art references which, in their opinion as one skilled in the pipeline art, would anticipate or render obvious the novel method and apparatus of the instant invention; however, for purposes of fully developing the background of the invention and establishing the state of the requisite art, the following reference is set forth: "New Machine Trenches and Backfills Subsea Lines", Pipe Line Industry, October 1981.